In the photomechanical trade, a mask containing an image that is opaque to actinic radiation is used in preparing a printing plate of some kind, e.g., letterpress, lithographic, etc. The mask is used as a phototool in exposing a layer of photosensitive resist-forming material present on a metal or plastic plate or a photopolymer printing plate matrix. After the exposure the printing plate is formed by etching or liquid development depending on the system present. The image in the mask must be of the highest possible contrast, e.g., completely black in the image areas, and completely transparent (free from fog) in the unexposed areas. Silver halide film known as "litho" film is used to prepare the mask. When a litho film is exposed through a halftone screen and developed, it contains an image comprised of dots. The dots correspond to the areas of the film under the transparent areas of the halftone screen and are comprised of exposed and developed material. Silver halide films are expensive and require special red light handling.
It is therefore desirable to provide a photosensitive element of good processing latitude for forming a lithographic negative of high quality which can use a low coating weight of silver or other metals that form insoluble sulfides. It is also desirable that said photosensitive element, in one or two layer embodiments, be capable of forming the lithographic negative image by diffusion reaction rather than solvent development.